Pressure ratio sensing

ABSTRACT

A pressure ratio sensor for producing a signal proportional to the ratio of two pneumatic pressures derived from two probes and comprising a casing, a diaphragm dividing the casing into two chambers each having an inlet, a needle attached to the diaphragm and extending through one of the chambers and controlling, in accordance with the position of the diaphragm, the effective area of an outlet from that chamber, the signal pressure from one probe being applied directly to one of the inlets, the signal pressure of the other probe being multiplied by a known factor, the multiplied signal pressure being applied to the other inlet and the position of the needle producing a signal indicative of the ratio.

United States Patent 1151 3,675,225

Glaze 1 July 4, 1972 54] PRESSURE RATIO SENSING 3,466,642 9/1969Terramorsi ..340/242 2,981,058 4/1961 Reed ..73/407 PR x [72] 2:22;Glaze Bm'ley 3,289,479 12/1966 Tausch ..73/407 R [73] Assignee: H. M.Hobsou Limited, London, England Primary Examiner-John W. Caldwell Filed:Nov. 1969 Assistant Examiner-Robert J. Mooney AttorneyE. T. Le Gates andMartin Kirkpatrick [21] App]. No.: 875,949

[57] ABSTRACT [30] Foreign Application Priority Data A pressure ratiosensor for producing a signal proportional to the ratio of two pneumaticpressures derived from two probes Nov 12, 1968 Great Br1ta1n ..53,600/68and comprising a casing a diaphragm dividing the casing into twochambers each having an inlet, a needle attached to the [52] diaphragmand extending through one of the chambers and [51] [m C] 6 19/00controlling, in accordance with the position of the diaphragm, [58]Fie'ld R 407 the effective area of an outlet from that chamber, thesignal pressure from one probe being applied directly to one of the137/815 235/200 60/3928 inlets, the signal pressure of the other probebeing multiplied by a known factor, the multiplied signal pressure beingapplied [56] References cued to the other inlet and the position of theneedle producing a UNITED AT PATENTS signal indicative of the ratio.

3,384,884 5/ 1968 Var ..340/ 196 3 Claims, 3 Drawing Figures f i I J E14 25 F\ If 16 3 14 1 PRESSURE RATIO SENSING This invention provides apressure ratio sensor which produces a signal proportional to the ratioof two absolute pneumatic pressures.

An instance of the practical application of such a unit is in the airintake of the engine of a supersonic aircraft, in which case thepneumatic pressures to be compared will be derived from two pressureprobes which afford information as to the position of a shock wave atthe intake, the derived signal being utilized to actuate mechanism forchanging the geometry of the intake in the event of deviation of theshock wave from a desired position.

One form of apparatus which will afiord a signal representative of theratio of two pneumatic pressures is the air potentiometer which has cometo be known as a microjet and is illustrated diagrammatically in FIG. 1of the accompanying drawings.

This consists of a diaphragm subject at one side to one of the pressureP and attached to a needle 2, having a tapered end 8 which controls theeffective area of convergent-divergent outlet orifice 3 of area a,,. Airat the other pressure P flows through a fixed orifice 4 of area a and achamber 5 at the other side of the orifice to the outlet orifice 3.

The air pressure P in the chamber 5 can be determined from the followingconditions, T being the temperature of the air entering through theorifice 4 and Tthe temperature of the air in the chamber.

Mass flow into the chamber 5, m =K a T P f (P /P) Mass flow out of thechamber 5, m K a, T P, provided that the orifice 3 is choked But m m sothat, provided T T,

and since a =f X), where X is the displacement of the needle X =1" i/ Inthe steady state the needle 2 will have moved to a position in which P=P hence Accordingly, provided that the outflow velocity through theorifice 3 is at least equal to the local speed of sound, the position ofthe needle 2 will be a function of the pressure ration P /P and can beutilized, e.g. by means of a potentiometer, to generate an electricalsignal proportional to P lP Unfortunately a microject cannot be usedsatisfactorily to give a signal proportional to the ratio P ll when theratio approximates to unity as is the case in the above-mentionedapplication to an air intake where the pressure ratio tends to vary overthe approximate range of 0.7 1.3.

According to the invention this difficulty is overcome by application ofthe pressure from one probe directly to the microjet and by providingfor pneumatic multiplication of the pressure sensed by the other probeby an accurately known factor prior to its application to the microjet.

The invention therefore provides a pressure ratio sensor for producing asignal proportional to the ratio of two pneumatic pressures derived fromtwo probes and comprising a casing, a diaphragm dividing the casing intotwo chambers, each having an inlet, a needle attached to the diaphragmand extending through one of the chambers and controlling, in accordancewith the position of the diaphragm, the efiective area of an outlet fromthat chamber, means for applying the signal pressure from one probedirectly to one of the inlets and means for multiplying the signalpressure from the other probe by an accurately known factor and applyingsaid multiplied signal pressure to the other inlet.

Apparatus for achieving such multiplication is shown in FIG. 2 of thedrawings. As there shown the signal pressure P from one of the probes(not shown) is passed through a pneumatic device 20 to a high gainpneumatic amplifier 21, from the output of which air at pressure P isfed back through an air potentiometer 22 having a choked outlet. Underthese conditions the pressure P, tapped from the potentiometer 22 willbe an exact fraction aP of the pressure P a being less than unity,determined by the areas of the two restrictors 6 and 7 in thepotentiometer. The device 20 subtracts the pressure P,, from the signalpressure P, and applies to the input of the amplifier 21 an air pressureequal to P P If G be the gain of the amplifier If therefore G issufficiently large, P l/a P and a can be so selected that P is, forexample, three times P,.

For high accuracy the gain of the amplifier must be high and thefeedback flow from the air potentiometer must be a small fraction of theflow through the potentiometer if sonic velocity is not attained in thesignal flow.

Suitable pneumatic pressure multiplying mechanism for achieving theresult indicated in FIG. 2 is described in our copending US. Pat.Application Ser. No. 875,934, filed Nov. 12, I969, now US. Pat. No.3,598,137, issued Aug. l0, I971.

A practical form of pressure ratio sensor according to the invention isshown in FIG. 3 of the accompanying drawings, in which partscorresponding to parts in FIG. I bear the same reference numerals.

As there shown, a diaphragm 1 divides a casing 30 into two chambers 31and 5 and the signal pressure P, derived from one of the probes isapplied through a union 23 to the upper surface of the diaphragm l. Thetapered end 8 of the needle 2 carried by the diaphragm 1 extends intothe convergent-divergent orifice 3. An extension of the needle is guidedby a sealing bush 13. The multiplied other signal pressure P is appliedto the underside of the diaphragm 1 through a union l6 and an orifice24, the area of which can be adjusted by a needle 25. After passagethrough the orifice 3 the air leaves through a union 14.

The needle carries the iron armature 18 of a linear transducer 10. Anoscillator/demodulator assembly 12 converts a DC. voltage applied to aconnector 11 into a 10 KC/s square wave which energizes the primarywinding of a transformer 9. The output in the secondary winding of thetransformer, which is determined by the position of the armature l8 andtherefore of the needle 2, is demodulated to supply a differential DC.output signal.

The pressure ratio range can be modified by mechanical adjustment of oneor both of the restrictors 6, 7 in the air potentiometer 22 FIG. 2

What I claim as my invention and desire to secure by Letters Patent is:

1. A pressure ratio sensor for producing a signal proportional to theratio of two pneumatic pressure signals derived from two probes andcomprising:

a casing;

a diaphragm dividing the casing into two chambers, each having a inlet;

a needle attached to the diaphragm and extending through one of thechambers, said needle controlling, in accordance with the position ofthe diaphragm, the effective area of an outlet from that one chamber;

means for applying the signal pressure from one probe directly to one ofthe inlets;

means for multiplying the signal pressure from the other probe by anaccurately known factor and applying said multiplied pressure to theother inlet; and,

signal producing means for producing a signal indicative of the positionof the needle.

2. A sensor as claimed in claim 1, in which the multiplying meanscomprises a high gain pneumatic amplifier having, connected to itsoutput, an air potentiometer containing two 3. A sensor as claimed inclaim 1 in which the needle carries the armature of a transformeradapted to generate an electrical signal indicative of the position ofthe needle and therefore of the ratio between the unmultiplied and themultiplied pressure signals.

1. A pressure ratio sensor for producing a signal proportional to theratio of two pneumatic pressure signals derived from two probes andcomprising: a casing; a diaphragm dividing the casing into two chambers,each having a inlet; a needle attached to the diaphragm and extendingthrough one of the chambers, said needle controlling, in accordance withthe position of the diaphragm, the effective area of an outlet from thatone chamber; means for applying the signal pressure from one probedirectly to one of the inlets; means for multiplying the signal pressurefrom the other probe by an accurately known factor and applying saidmultiplied pressure to the other inlet; and, signal producing means forproducing a signal indicative of the position of the needle.
 2. A sensoras claimed in claim 1, in which the multiplying means comprises a highgain pneumatic amplifier having, connected to its output, an airpotentiometer containing two restrictors in series, and a pneumaticdevice, connected to the input of the amplifiers and to a tapping in theair potentiometer between the restrictors, which subtracts from thesignal pressure to be multiplied the pressure at the tapping in the airpotentiometer and applies this difference in air pressure to the inputof the amplifier.
 3. A sensor as claimed in claim 1 in which the needlecarries the armature of a transformer adapted to generate an electricalsignal indicative of the position of the needle and therefore of theratio between the unmultiplied and the multiplied pressure signals.